This was the first flight after a maintenance inspection. The flaps were left full down after a post-inspection by company mechanics, before the flight. Witnesses stated the flaps were down when the aircraft taxied to takeoff on RWY 19L. Witnesses stated the aircraft climbed steeply after it lifted off with a pitch attitude up to 60 degrees. According to witnesses, the aircraft climbed to 200 to 500 ft AGL, before it stalled and descended nose down in a left turn. The aircraft crashed about 300 ft left of and 2000 ft down the runway. Examination of the aircraft revealed the flaps were full down. The scroll type checklist was positioned at the beginning of the takeoff check. The flight manual recommends a 10 degree flap setting for takeoff and prohibits flap full down takeoff. Review of previous DHC-6 accidents involving flap full down resulted in a steep takeoff climb and excessive pitch attitude followed by a stall. The position of the control lock suggests it may have been in the locked position during the takeoff.